J. Phys. Chem. C 2007, 111, 7049-7057
7049
Reaction of CH3OH on Pd/ZnO(0001) and PdZn/ZnO(0001) Model Catalysts Parthasarathi Bera and John M. Vohs* Department of Chemical and Biomolecular Engineering, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6393 ReceiVed: December 11, 2006
The adsorption and reaction of CH3OH on Pd and PdZn films and particles supported on the (0001) surface of a ZnO single crystal were studied using temperature-programmed desorption (TPD). At 300 K, vapordeposited Pd films followed a two-dimensional island growth mode and preferentially blocked the active sites for dissociative adsorption of methanol on the ZnO(0001) surface. Upon heating, the Pd films agglomerated into particles and reacted with the ZnO substrate to form a PdZn alloy. Methanol was found to undergo complete dehydrogenation on the supported Pd and PdZn films and particles producing CO and H2. Consistent with previous studies, alloying of Pd with Zn decreased the strength of the interaction of CO with the metal surface, as evidenced by a decrease in the CO desorption temperature. A low-temperature pathway for the oxidation of CO to CO2 on Pd/ZnO(0001) and PdZn/ZnO(0001), in which oxygen was supplied by the ZnO(0001) support, was also identified.
Introduction Due to its high hydrogen/carbon ratio, low sulfur content, and the ability to be steam re-formed to hydrogen and carbon dioxide under relatively mild conditions, methanol has emerged as an attractive liquid fuel for polymer exchange membrane fuel cell (PEMFC) systems.1-5 Using methanol for this purpose, however, requires the development of stable and highly active methanol steam re-forming (MSR) catalysts that have high selectivity to CO2 relative to CO. To date, the most studied MSR catalysts are those based on Cu, such as Cu/ZnO.6-16 While these Cu-based catalysts are highly active and selective, they have the drawback of being prone to sintering at relatively moderate temperatures and are pyrophoric once reduced. Recently, there has been interest in Pd supported on ZnO as an alternative MSR catalyst, which does not have some of the problems associated with the Cu-based systems.1,17-19 In general, supported Pd is a relatively poor catalyst for MSR, producing predominantly CO and H2.20 This is due to the fact that methanol readily decomposes to CO and H2 on bulk Pd at temperatures below 300 K.21,22 Iwasa et al. were the first to observe, however, that Pd supported on ZnO has unique catalytic properties and is highly active for the MSR, with the selectivity to CO2 being greater than 97%.23-26 Modification of the catalytic properties of Pd in the presence of ZnO and the high activity and selectivity of Pd/ZnO for MSR has been ascribed to the formation of a PdZn alloy.24,27 While the importance of the PdZn alloy has been firmly established, many questions regarding how Zn modifies the catalytic activity of the Pd, the identity of the reaction intermediates and mechanism, and the role of the support remain unanswered. In an effort to provide a more fundamental understanding of the relationships between the structure and MSR activity of Pd/ ZnO catalysts, we are studying the structure and reactivity of model catalysts consisting of Pd and PdZn films and particles supported on the (0001) surface of a ZnO single crystal. In a recent paper, we reported a study of the structure and thermal * To whom correspondence should be addressed. E-mail: vohs@ seas.upenn.edu.
stability of submonolayer, monolayer, and multilayer Pd films on ZnO(0001) and the reaction of the Pd with the ZnO surface to form a PdZn alloy.28 In that study, it was determined, using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED), that Pd films deposited from the vapor phase onto a ZnO(0001) surface held at 300 K follow a twodimensional island (2DI) growth mode in which 2D metal islands are formed up to a critical coverage of ∼0.5 ML, at which point growth occurs primarily in a layer-by-layer fashion on top of the islands. The Pd films were found to be metastable and agglomerated into particles upon heating. Reaction of the Pd with the ZnO(0001) surface to form a PdZn alloy was also found to occur upon heating above only 700 K in the relatively moderate reducing environment of ultrahigh vacuum. In the work reported here, we have extended our previous studies of the structure of model Pd/ZnO(0001) catalysts to include the adsorption and reaction of methanol on these catalysts. While it would be useful to spectroscopically probe the intermediates formed by reaction of methanol on the Pd/ZnO(0001) surface, the dielectric and phonon properties of the single-crystal ZnO support severely complicate the application of standard surface vibrational spectroscopies, such as HREELS and reflectionadsorption infrared spectroscopy, to characterize surface intermediates on the Pd/ZnO(0001) support. We have, therefore, in this study, chosen to use temperature-programmed desorption (TPD) as the primary means to characterize reaction pathways for adsorbed methanol and to determine how interactions of the Pd with the ZnO(0001) surface influence reactivity. As will be shown below, the TPD data obtained in this study further demonstrate the effect of alloying of Pd with Zn on the interaction of CO with the supported metal and have allowed a new pathway for the production of CO2 during the reaction of methanol on Pd/ZnO to be identified. Experimental Methods All experiments were conducted in two separate ultrahigh vacuum (UHV) surface analysis chambers with background pressures of ∼2 × 10-10 Torr. Both chambers were equipped
10.1021/jp068501f CCC: $37.00 © 2007 American Chemical Society Published on Web 04/26/2007
7050 J. Phys. Chem. C, Vol. 111, No. 19, 2007 with an ion sputter gun (Physical Electronics) for sample cleaning, a quadrupole mass spectrometer (UTI) for TPD studies, and a quartz crystal film thickness monitor (Maxtek, Inc.) for measuring the metal flux from the evaporative palladium metal source. One of the chambers contained an LK Technologies model 3000 HREEL spectrometer and a retarding field electron energy analyzer with coaxial electron gun (OCI Vacuum Microengineering) that was used for both Auger electron spectroscopy (AES) and LEED. The other chamber contained a hemispherical electron energy analyzer (Leybold-Heraeus) and an X-ray source (VG Microtech), which were used for XPS. The ZnO(0001) single-crystal substrate used in this study was approximately 6 mm × 6 mm × 1 mm in size and mounted in a tantalum foil holder that was attached to the sample manipulator on the UHV chamber. The sample temperature was monitored using a chromel-alumel thermocouple that was attached to the back surface of the ZnO crystal using a ceramic adhesive (Aremco). The sample was heated via conduction from the resistively heated tantalum foil holder. The ZnO(0001) surface was cleaned using repeated cycles of sputtering with 2 kV of Ar+ ions followed by annealing at 875 K. The sputter/ anneal cycles were repeated until the surface was free from carbon and other impurities, as determined by AES or XPS. An evaporative metal source was used for Pd deposition and was constructed by wrapping 0.127 mm diameter high-purity Pd wire (Alfa Aesar, 99.9%) around a 0.2 mm diameter tungsten wire. The tungsten wire was then attached to an electrical feedthrough on the UHV chamber, allowing it to be heated resistively. The steady-state flux of Pd was monitored using a quartz crystal oscillator. The typical growth rate was ∼0.2 Å s-1. The ZnO substrate was held at 300 K during Pd deposition. Throughout this paper, we report the Pd coverage in equivalent monolayers (ML), where 1 ML of Pd is defined to be 1.53 × 1015 atoms cm-2, which is the packing density in the Pd(111) surface. The quadrupole mass spectrometer on each chamber was enclosed in a quartz glass tube that was closed at one end except for a 5 mm diameter aperture. The ZnO sample was placed in front of this aperture during TPD experiments. All TPD experiments were performed using a sample heating rate of 2 K s-1. The liquid CH3OH (Aldrich, 99.99%) was purified using repeated freeze-pump-thaw cycles prior to use. CH3OH was introduced into the vacuum chambers using variable leak valves, and the sample was exposed to 0.5 L of CH3OH at 300 K in all TPD experiments. In some cases, methanol that was labeled with O18 (Alfa Aesar) was used in the TPD experiments. Results Growth and Characterization of Pd Films on ZnO(0001). The growth of Pd films on ZnO(0001) has been characterized by HREELS, XPS, and LEED and described in detail previously.28 As noted above, in our previous study, it was observed that Pd deposited from the vapor phase on ZnO(0001) at 300 K proceeded via a two-dimensional island (2DI) growth mode in which Pd islands were formed up to a coverage of ∼0.5 monolayers, at which point additional growth occurred on top of the metal islands. A similar growth mode has been reported for Cu and Pt films on ZnO surfaces.29-33 Pd films produced in this manner are not thermally stable and undergo partial agglomeration upon heating, indicating that Pd interacts rather weakly with the ZnO(0001) surface. XPS provided clear evidence for the formation of a PdZn alloy upon annealing of Pd films on ZnO(0001) at 700 K.28 This is demonstrated by the Pd(3d) XP spectra for a 0.5 ML Pd film on ZnO(0001) as
Bera and Vohs
Figure 1. Pd(3d) XPS spectra obtained from a 0.5 ML Pd/ZnO(0001) sample as a function of the annealing temperature.
a function of annealing temperature displayed in Figure 1. Note that the spectrum of the Pd film heated to 500 K contains a single 3d5/2,3/2 spin-orbit doublet with the peaks centered at 335.0 and 340.2 eV corresponding to Pd metal, whereas the spectra obtained after heating to 800 and 900 K contain additional features on the high-binding-energy sides of each peak centered at approximately 336.3 and 341.4 eV. On the basis of comparison to the work of Rodriguez,34 who used XPS to characterize PdZn films deposited on Ru(001), these higher binding energy peaks are consistent with alloying of a portion of the Pd with Zn. It should also be noted that XPS studies of high-surface-area Pd/ZnO catalysts have shown that upon reduction in H2, formation of the PdZn alloy can occur at temperatures as low as 420 K.27 CH3OH TPD Studies. ZnO(0001). Before discussing the reaction of CH3OH on ZnO(0001)-supported Pd films, it is useful to consider the reaction of CH3OH on the clean ZnO(0001) surface. Figure 2A displays TPD data obtained from ZnO(0001) dosed with 0.5 L of CH3OH at 300 K. These data are similar to those in previous studies of this system.35-39 These previous studies have shown that methanol adsorbs dissociatively on exposed cation-anion site pairs on ZnO(0001) at room temperature to produce adsorbed methoxide (CH3O) and hydroxyl (OH) intermediates. The methoxide species undergo dehydrogenation at 525 K, producing gaseous formaldehyde (H2CO). In parallel with this reaction, a portion of the adsorbed methoxides are oxidized by lattice oxygen to adsorbed formates (HCOO). These formate species decompose at 575 K, producing gaseous CO, CO2, and H2O. 0.35 ML Pd/ZnO(0001). TPD spectra obtained from a CH3OH-dosed, freshly prepared 0.35 ML Pd film on ZnO(0001) are displayed in Figure 2B. These spectra are similar to those from the clean ZnO(0001) surface and contain peaks for H2CO
Reaction of CH3OH on Pd/ZnO(0001) and PdZn/ZnO(0001)
J. Phys. Chem. C, Vol. 111, No. 19, 2007 7051
Figure 2. CH3OH TPD spectra obtained from (A) ZnO(0001) and (B) 0.35 ML Pd/ZnO(0001).
at 525 K, and CO, CO2, and H2O at 575 K, which can be assigned to the dehydrogenation of adsorbed methoxides and the decomposition of adsorbed formates, respectively, on the exposed portions of the ZnO(0001) surface. An additional, broad CO desorption peak centered at 470 K is present in the spectra from the Pd/ZnO(0001) sample that was not observed for the clean ZnO(0001) surface. This peak temperature corresponds to that reported in the literature for the desorption of CO from Pd(111).40,41 Previous studies have shown that methanol decomposes to CO and H on Pd(111) below room temperature.21 Thus, the CO peak at 470 K in the spectrum from the 0.35 ML Pd/ZnO(0001) sample can be assigned to desorption of CO produced by dehydrogenation of CH3OH on the Pd film. Since hydrogen desorbs from Pd below room temperature, the hydrogen produced by this reaction most likely desorbed while dosing CH3OH at 300 K. Figure 3 displays CO desorption spectra obtained in CH3OH TPD experiments from the freshly prepared 0.35 ML Pd/ZnO(0001) sample and after annealing this sample for 10 min at 800, 825, and 850 K. For comparison, the CO desorption spectrum from the clean ZnO(0001) surface is also included in the figure. Note that the intensity of the CO desorption peak at 575 K due to formate decomposition on ZnO(0001) increases with annealing temperature and, after heating to 850 K, is essentially the same size as that obtained from the clean ZnO(0001) surface. Desorption spectra for H2CO and CO2 were also collected and contained only peaks indicative of the reaction of methanol on ZnO(0001). The trends in the intensities for these peaks were the same as that for the CO peak at 575 K. These results are consistent with the XPS and HREELS characterization study28 described above, which shows that the Pd film agglomerates upon heating, thereby exposing more of the ZnO(0001) surface. The trends in the desorption of CO produced by decomposition of CH3OH on the Pd film are more complex. As noted above for the as-deposited 0.35 ML Pd film, the CO peak centered at 470 K is consistent with that reported for bulk Pd. Annealing to 800 and then 825 K caused the intensity of this peak to increase somewhat and the peak maximum to shift from
Figure 3. CO TPD spectra from CH3OH-dosed 0.35 ML Pd/ZnO(0001). The spectra correspond to a sample with a freshly deposited Pd film and after annealing this sample to the indicated temperatures for 10 min. For comparison purposes, the CO desorption spectrum from CH3OH-dosed ZnO(0001) is also included in the figure.
470 to 450 K. The low-temperature CO peaks from the annealed samples are also broader than that obtained from the as-deposited Pd film. Annealing to 850 K caused the intensity of the CO desorption peak from the supported Pd to decrease, consistent with agglomeration of the metal layer, and to shift further down in temperature to 435 K. The morphology of the metal layer and its composition, due to alloying with Zn, are changing in this series of experiments. The observed changes in the CO desorption spectra from the supported metal are due to both of these effects. Note, however, that previous studies of Pd and PdZn alloys supported on Ru(001) have shown that alloying of Pd with Zn weakens the interaction of CO with the metal and
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Figure 4. (A) CO and (B) CO2 TPD spectra from CH3OH-dosed 0.5 ML Pd/ZnO(0001). The spectra correspond to a sample with a freshly deposited Pd film and after annealing this sample to the indicated temperatures for 10 min. The CO2 spectra are scaled by a factor of 5.3 relative to the CO spectra.
causes the CO desorption temperature to decrease.34 Thus, the observed decrease in the CO desorption temperature from the Pd film on ZnO(0001) can be attributed to formation of the PdZn alloy. 0.5 ML Pd/ZnO(0001). CO and CO2 desorption spectra obtained from a CH3OH-dosed 0.5 ML Pd/ZnO(0001) sample as a function of pretreatment conditions are shown in Figure 4A and B, respectively. The CO desorption spectrum for the as-deposited Pd film and that after annealing at 750 K for 10 min both contain only the low-temperature CO desorption peak centered at ∼450 K, which corresponds to desorption of CO produced by methanol decomposition on the Pd. (The rising signal at high temperatures in the spectrum from the as-deposited film is due to desorption of CO from the sample support hardware.) The trends in the intensity of this peak and its position with annealing temperature are similar to that observed for the 0.35 ML Pd/ZnO(0001) sample. Note that a decrease in the peak temperature is again observed upon annealing to higher temperatures, with the peak maximum decreasing from 450 K for the as-deposited Pd film to 430 K after annealing at 850 K for 10 min. This shift can again be attributed to formation of a PdZn alloy. The data in Figure 4A also show that the 0.5 ML Pd film completely suppresses the reaction of methanol on the ZnO(0001) surface. This is evident by the absence of a CO desorption peak at 575 K resulting from the decomposition of formate species on the ZnO for samples heated to 750 K or less. This peak does start to emerge, however, upon annealing the sample at 800 K and is clearly evident in the spectrum obtained from the sample annealed at 850 K. The emergence of this peak is consistent with agglomeration of the Pd layer upon heating, thereby exposing sites on the ZnO(0001) surface that are active for the dissociative adsorption of methanol and its subsequent oxidation. Figure 4B displays CO2 desorption spectra obtained during the CH3OH TPD experiments with the annealed samples in Figure 4A. All of the CO2 desorption spectra contain a peak at 575 K. Since this peak temperature is the same as that for CO2 produced by decomposition of formate intermediates on ZnO(0001), it is tempting to assign it to this reaction. This
Bera and Vohs
Figure 5. (A) CO and (B) CO2 TPD spectra from CH3OH-dosed 0.75 ML Pd/ZnO(0001). The spectra correspond to a sample with a freshly deposited Pd film and after heating the sample briefly to the indicated temperatures or annealing for 10 min at the indicated temperatures. The CO2 spectra are scaled by a factor of 3.7 relative to the CO spectra.
assignment is consistent with the CO TPD data for the sample annealed at 850 K, which contain a CO desorption peak of similar intensity at this temperature. The lack of a coincident CO peak of similar intensity for the samples annealed at 750 and 800 K suggests, however, that, at least for these two samples, the CO2 peak may have a different origin. It should also be noted that the peak shape is not consistent with that observed for formate decomposition on ZnO(0001). As shown in Figure 2A, the CO and CO2 peaks at 575 K, resulting from decomposition of formate species on ZnO(0001), have the expected shape for a first-order process. In contrast, for the 0.5 ML Pd sample annealed at 750 and 800 K, broad tails are readily apparent on the low-temperature sides of the CO2, peaks suggesting the possibility of a second lower temperature CO2 desorption feature for these samples. The CO2 peak sharpened considerably upon annealing the sample at 850 K. For this sample, the peak shape is more consistent with that obtained for formate decomposition on ZnO(0001). Even for this sample, however, a broad low-temperature tail is still present, although its intensity is less than that in the data obtained from the samples annealed at lower temperatures. Since the extent of agglomeration of the Pd layer increases with annealing temperature, the decrease in the intensity of the low-temperature CO2 desorption features upon annealing to high temperatures is consistent with this feature resulting from reaction on the metal. 0.75 ML Pd/ZnO(0001). Figure 5A and B displays CO and CO2 desorption spectra obtained during CH3OH TPD experiments with a 0.75 ML Pd/ZnO(0001) sample as a function of pretreatment conditions. The CO spectrum for the freshly prepared 0.75 ML Pd film contains a single peak at 450 K, which can again be attributed to desorption of CO produced by CH3OH decomposition on the Pd. Consistent with the data obtained for lower metal coverages, the trend in the intensity of this peak as a function of the sample annealing temperature is complex, and it increases dramatically upon annealing the sample at 800 K and then decreases upon annealing to higher temperatures. The position of this peak also decreases upon
Reaction of CH3OH on Pd/ZnO(0001) and PdZn/ZnO(0001)
Figure 6. (A) CO and (B) CO2 TPD spectra from CH3OH-dosed 1.5 ML Pd/ZnO(0001). The spectra correspond to a sample with a freshly deposited Pd film and after heating the sample briefly to the indicated temperatures or annealing for 10 min at the indicated temperatures. The CO2 spectra are scaled by a factor of 3 relative to the CO spectra.
annealing above 800 K and shifts from 450 K for the as-deposited Pd film to 425 K for the 850 K-annealed film. As noted above, this shift is consistent with the formation of a PdZn alloy. A CO peak at 575 K, which is characteristic of formate decomposition on ZnO(0001), was not observed for the 0.75 ML Pd sample for annealing temperatures less than 800 K, demonstrating that the Pd film completely poisons the sites for dissociative adsorption of CH3OH on ZnO(0001). This peak emerges, however, upon agglomeration of the metal, which is significant at temperatures above 800 K. Note that the additional smaller m/z 28 peak centered at 525 K in the data from the 850 K-annealed sample is a cracking fragment of H2CO, which is produced on the exposed portions of the ZnO(0001) substrate that has not been subtracted from the spectrum. The CO2 desorption spectra obtained from the CH3OH-dosed 0.75 ML Pd/ZnO(0001) sample in Figure 5B are complex. For the as-deposited Pd film, the CO2 spectrum contains a small peak centered at 570 K. As was the case for the 0.5 ML Pd sample, the lack of a coincident CO peak at this temperature suggests that this feature may not be due to formate decomposition on the ZnO(0001) surface. Briefly heating in stages up to 800 K caused the CO2 peak to broaden considerably and the peak maximum to shift down to 510 K. The peak intensity also increased during this series of experiments in a manner similar to that of the CO desorption peak from the Pd. This result again suggests that the CO2 peak is due to reaction on the metal. The top three spectra in the figure were obtained after annealing the sample for 10 min at 800, 825, and 850 K. In this series of spectra, a CO2 peak resulting from formate decomposition on the ZnO(0001) surface emerges as the metal layer agglomerates, while the CO2 peak resulting from reaction on the metal continues to shift to lower temperatures and decreases significantly in intensity. 1.5 ML Pd/ZnO(0001). Figure 6A and B contains a similar data set for CO and CO2 desorption obtained during TPD with a CH3OH-dosed 1.5 ML Pd/ZnO(0001) sample. For this sample, the trends in the CO desorption data are similar to those obtained from the samples with lower metal coverages. For the as-
J. Phys. Chem. C, Vol. 111, No. 19, 2007 7053
Figure 7. TPD spectra from CH318OH-dosed 1.5 ML Pd/ZnO(0001) samples annealed for 10 min at (A) 800 and (B) 900 K. Spectra are shown for the following m/z ratios: 28 (C16O), 30 (C18O), 44 (C16O16O), and 46 (C16O18O).
prepared 1.5 ML Pd film, the CO desorption peak is centered at 460 K, as expected for desorption of CO from Pd. Upon annealing to higher temperatures, the Pd-CO peak shifts to lower temperatures, consistent with alloying of the Pd with Zn from the substrate. After annealing at 850 K, the Pd-CO peak is centered at 440 K. The CO peak due to formate decomposition on ZnO also emerges upon annealing above 800 K, signifying agglomeration of the metal layer. The trends in the CO2 desorption data from the 1.5 L Pd/ ZnO(0001) sample are similar to those for the 0.75 ML Pd/ ZnO(0001) sample. A broad CO2 peak between 450 and 600 K is present in the spectra from samples that had been annealed up to 825 K and, with the exception of a lower peak temperature for the as-deposited film, the maximum in the CO2 desorption signal decreases with an increasing annealing temperature. A CO2 peak resulting from formate decomposition on the ZnO(0001) surface also grows in upon annealing to 825 and 850 K. In order to elucidate the origins of the various CO2 desorption features in the TPD data from the Pd-covered samples, a series of TPD experiments was performed with a 1.5 ML Pd/ZnO(0001) sample using CH3OH labeled with 18O. Figure 7 displays TPD results obtained from a 1.5 ML Pd/ZnO(0001) sample that has been annealed for 10 min at 800 (panel A) and at 900 K (panel B). A 1.0 L CH318OH dose was used in these experiments. The figure contains desorption curves for m/z ) 28 (C16O), 30 (C18O), 44 (C16O16O), and 46 (C16O18O). Spectra for m/e 48 (C18O18O) were also collected but were featureless and are not included in the figure. The overall trends for the CO and CO2 peaks as a function of annealing temperature are consistent with those in Figure 6. The low-temperature CO product contains 18O, although some 16O is also present. The amount of C16O produced is somewhat greater than that expected based on the isotopic composition of the CH3OH reactant as reported by the manufacturer (95% 18O) but is consistent with the cracking pattern measured for the labeled methanol using the mass spectrometer on our system. The isotopic composition of the high-temperature CO peak is different than that of the low-temperature peak and contains predominantly 16O. As described above, this peak is due to decomposition of formate species produced by reaction of methanol on the ZnO(0001) surface. The fact that the product
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contains predominantly 16O demonstrates that exchange of oxygen between adsorbed formates and the ZnO lattice is facile. Note that the CO2 peak at 575 K, which also results from formate decomposition on the ZnO(0001) surface, is predominately C16O16O. This observation also supports the conclusion that formates exchange oxygen with the ZnO lattice. The data in Figure 7 also show that the CO2 that desorbs between 400 and 520 K is composed almost entirely of C16O18O. Thus, in this temperature range, the CO2 contains one oxygen from the methanol reactant and one oxygen from the ZnO lattice. Discussion The detailed TPD results obtained in this study show that one of the effects of the vapor-deposited Pd is to preferentially block the active sites for dissociative adsorption and subsequent oxidation of methanol on the ZnO(0001) surface. To explain this poisoning effect of Pd, it is useful to consider the accepted mechanism for the reaction of methanol on ZnO(0001), which is as follows35,38
above ∼150 K CH3OH(g) f CH3O(ad) + H(ad)
(1)
above 400 K CH3O(ad) + O(l) f HCOO(ad) + 2 H(ad) (2) 510-530 K CH3O(ad) f H2CO(g) + H(ad)
(3)
560-580 K HCOO(ad) f CO2(g) + H(ad)
(4)
HCOO(ad) f CO (g) + OH(ad)
(5)
2 OH(ad) f H2O(g) + O(l)
(6)
(The adsorbed H atoms produced in this sequence either desorb as H2 or react with lattice oxygen to produce additional water. O(l) stands for lattice oxygen.) The initial step in the mechanism (reaction 1) is the dissociative adsorption of methanol to form absorbed methoxide intermediates and hydroxyl groups. On the basis of the observation that dissociative adsorption of methanol occurs on the Znterminated ZnO(0001) surface and not on the O-terminated ZnO(0001h) surface, it has been concluded that exposed cationanion site pairs are the active sites for this reaction.35,38 As shown in Figure 8, which displays ideal bulk termination models of these surfaces, due to the differences in the size of the Zn2+ and O2- ions, both ions are accessible to adsorbates on ZnO(0001), and only the O2- anions are accessible on ZnO(0001h). While this explanation is plausible, it should be noted that the O2- ions on the ZnO(0001h) surface are fully coordinated and, therefore, may not be highly reactive toward adsorbates. Diebold et al. have used STM to study the structure of ZnO(0001) and ZnO(0001h) surfaces prepared by sputtering and annealing in vacuum42,43 and found that the (0001) surface is covered with single-layer, high, triangular-shaped islands, giving this surface a high concentration of steps that are terminated by under-coordinated oxygen anions. Step edges were also observed on the (0001h) surface but with a much lower concentration. In light of these STM results, it has been suggested that cation-anion site pairs at step edges, for which each ion has a single coordination vacancy, are the active sites for dissociative adsorption of methanol and other Brønsted acids on the ZnO(0001) surface.37 The data obtained in the present study for Pd-covered surfaces are consistent with this model. Vapor-deposited Pd forms two-dimensional islands during the initial stages of film growth on ZnO(0001). Since adsorption
Figure 8. Models of the ZnO(0001)-Zn and ZnO(0001h)-O surfaces. The dark spheres represent Zn2+ cations, and the light spheres represent O2- anions.
at a step edge would maximize the number of nearest neighbors for an adsorbed Pd atom, the step edges are likely to be the nucleation sites for the metal islands.33,44-46 If this is the case and if the step edges are also the active sites for methanol adsorption, the Pd film would preferentially poison the dissociative adsorption of methanol on this surface. The data in Figures 2 and 4 show that this is indeed the case. Note that only 0.5 ML of Pd is required to completely suppress dissociative adsorption of methanol, as evidenced by the lack of formate decomposition products at 575 K. It should also be noted that this result is consistent with a previous study of Pt films on ZnO(0001), where it was found that vapor-deposited Pt preferentially poisoned the methanol adsorption sites.31 In addition to preferentially blocking the sites for dissociative adsorption of methanol on ZnO(0001), submonolayer Pd films influence the selectivity of reactions occurring on the exposed portions of the ZnO surface. This is particularly noticeable for the decomposition of formate intermediates (reactions 4 and 5) that are produced by oxidation of adsorbed methoxides (reaction 2). As shown in Figure 2A, this reaction occurs at 575 K, producing CO, CO2, and H2O with a CO to CO2 ratio of approximately 3:1. Note, however, that for the fresh 0.35 ML Pd/ZnO(0001) sample (see Figure 2B), this ratio decreases to only 1.7:1. An increase in the selectivity for dehydrogenation of adsorbed formates to produce CO2 at the expense of dehydration to produce CO has also been previously reported for Pt-covered ZnO(0001).31 The mechanism by which small amounts of Pd or Pt influence in the relative rates of the dehydrogenation and dehydration of formate intermediates on ZnO(0001) is not clear based on the results obtained in these studies. As suggested by Grant et al., the fact that the formate decomposition peak temperature is the same on clean and metalcovered surfaces argues against an electronic effect.31 In their study of the reaction of methanol on Pt/ZnO(0001), Grant et al. also suggested that dehydration of formates may occur on
Reaction of CH3OH on Pd/ZnO(0001) and PdZn/ZnO(0001) oxygen vacancy defect sites on the ZnO(0001) surface and that these sites are preferentially blocked by the metal. While submonolayer coverages of Pd and Pt both have a similar effect on the selectivity for the decomposition of formate species on ZnO(0001), they appear to have a much different effect on the dehydrogenation of methoxide intermediates on this surface. As shown in Figure 2A, the dehydrogenation of methoxides on ZnO(0001) occurs at 525 K and produces gaseous formaldehyde (reaction 3). Note that a prominent H2CO peak at 520 K is also present in the TPD data from the 0.35 ML Pd sample in Figure 2B. In contrast, for Pt/ZnO(0001), Grant et al. reported that the formaldehyde peak disappeared completely upon deposition of only 0.01 ML of Pt.31 The reason for this difference in the effectiveness of Pd and Pt in suppressing the methoxide dehydrogenation activity is not clear, but it may be due to differences in the nucleation of the metal islands on the surface. Both Pt and Pd films undergo twodimensional island growth on ZnO(0001). The critical island coverage at which growth on top of the monolayer islands starts to dominate is higher for Pt31 compared to that for Pd,28 suggesting that Pt interacts more strongly with the surface. Let us now turn our attention to the reaction of methanol on the supported Pd and PdZn films and particles. Previous studies have shown that the primary reaction pathway for CH3OH on Pd(111) is sequential dehydrogenation to CO and H.21,22,47-49 During a TPD experiment, this pathway proceeds as follows
175-250 K CH3OH(g) f CH3O(ad) + H(ad)
(7)
250-300 K CH3O(ad) f CHxO(ad) + H(ad)
(8)
CHxO(ad) f CO(ad) + H(ad) H(ad) + H(ad) f H2(g) 450-500 K CO(ad) f CO(g)
(9) (10) (11)
In the present study, the Pd-covered ZnO(0001) samples were exposed to methanol at room temperature, and all but the last of these reactions occurred while dosing. In the TPD data from all the as-deposited Pd films, the desorption of CO produced by dehydrogenation of methanol on the metal occurred with a peak temperature between 450 and 470 K, which is in the range reported for Pd(111).40,41 Several trends are apparent in the CO TPD peaks from the supported Pd layers as the sample is annealed to higher temperatures. First, the intensity of the CO desorption peak changes in a complex manner with annealing temperature, but it generally increases for annealing temperatures up to 800 K and then decreases upon annealing at higher temperatures. This trend is most apparent in the TPD data for the 0.75 and 1.5 ML Pd coverages. In our previous study of the structure of Pd films on ZnO(0001), it was found that the 2D islands formed during film growth at 300 K start to agglomerate upon heating to relatively low temperatures, but this process is greatly accelerated at temperatures above 800 K. Thus, the decrease in the Pd-CO peak intensity upon annealing to high temperatures can be attributed to a loss of surface area accompanying agglomeration of the metal film into particles. The increase in the PdCO peak intensity at intermediate annealing temperatures observed for the 0.75 and 1.5 ML Pd/ZnO(0001) samples is more difficult to explain but may also be related to changes in the morphology of the metal film which cause an increase in the reactive sticking coefficient for methanol on the metal. This might occur if agglomeration of the metal film produces an increase in the number of step or defect sites. Since the Pd
J. Phys. Chem. C, Vol. 111, No. 19, 2007 7055 reacts with the ZnO substrate to form a PdZn alloy upon heating, changes in the surface composition of the metal may also play a role in effecting the amount of CO produced on the metal. The second trend apparent in the Pd-CO desorption data is that the peak temperature decreases with increasing annealing temperature. As demonstrated by the XPS data in Figure 1 for a 0.5 ML Pd/ZnO(0001) sample, PdZn alloy formation occurs upon heating to 700 K. Rodriguez has studied, in detail, the interaction of CO with Pd and PdZn alloy films supported on Ru(001) and found that the electronic interactions between Pd and Zn weaken the Pd(4d)-CO(2π) bond.34 This causes a dramatic decrease in the CO desorption temperature, which shifts from 460 K for a pure Pd film to 300 K for a film with a PdZn ratio of 1:0.85. On the basis of comparisons to Rodriguez’s results, it can be concluded that the downward shift in the CO desorption temperature, which accompanies annealing of the Pd/ZnO(0001) samples to temperatures above 700 K, is due to PdZn alloy formation. The magnitude of the shift in the CO desorption temperature for the annealed Pd/ZnO(0001) samples was between 20 and 30 K depending on the Pd coverage. On the basis of Rodriguez’s CO desorption data, this magnitude of a shift corresponds to films containing 10-20% Zn. This estimate is in reasonable agreement with the XPS data for the 0.5 ML Pd/ZnO(0001) sample in Figure 1. Perhaps the most interesting result obtained in this study is the observation of a low-temperature pathway for the oxidation of adsorbed methoxide intermediates to CO2 on the Pd/ZnO(0001) and PdZn/ZnO(0001) samples. For the 0.5, 0.75, and 1.5 ML Pd/ZnO(0001) samples, CO2 desorption features were observed in the CH3OH TPD data that were distinct from the CO2 product resulting from decomposition of formate intermediates on the exposed portions of the ZnO(0001) surface. For as-deposited Pd films and samples that had been heated to only 800 K, this second CO2 peak was centered between 520 and 575 K. Upon annealing the Pd/ZnO(0001) samples at 800 K or above, which causes incorporation of Zn into the Pd film, the peak position shifted to lower temperatures, and for samples annealed at 850 K, the CO2 product appeared as a broad desorption feature between 400 and 510 K. The intensity of this CO2 peak as a function of the sample annealing temperature was similar to that of the CO peak from the metal and increased for intermediate annealing temperatures and decreased upon annealing at high temperature and agglomeration of the metal film. The intensity of the peak also scaled with the metal coverage. Both of these observations indicate that the CO2 is produced by a reaction occurring on the metal. It is noteworthy that this pathway for the production of CO2 appears to be specific to Pd since it is not observed for Pt films on ZnO(0001).31 In order to determine the pathway for the production of CO2 at low temperatures on Pd/ZnO(0001), it is useful to know the origin of the oxygen atoms in this product. One of them clearly originates with the initial methanol reactant. There are two options, however, for the second oxygen, methanol or the ZnO lattice. The former is possible since low index planes of Pd exhibit some activity for the activation of methanolic C-O bonds.22,48 Thus, CO formed by dehydrogenation of methanol could react with oxygen formed by methanolic C-O bond activation to produce CO2. While this pathway is plausible, the TPD experiments using 18O-labeled methanol allow it and all other pathways involving reactions of only methanol-derived species to be ruled out. As shown in Figure 7, the isotopic composition of the lower-temperature CO2 product was pre-
7056 J. Phys. Chem. C, Vol. 111, No. 19, 2007
Bera and Vohs
dominantly C16O18O. This result demonstrates that one of the oxygen atoms in this CO2 product is supplied by the ZnO lattice. Zn2+ cations are reduced to metallic Zn during reaction of the supported Pd with the ZnO substrate to form the PdZn alloy. This reaction must also produce oxygen atoms that may end up adsorbed on the surface of the metal. Thus, for the TPD experiments with the annealed Pd/ZnO(0001) samples, it is possible that the metal was partially covered with oxygen prior to exposure to methanol. If this was the case, the lowtemperature CO2 product could result from reaction of adsorbed methoxide intermediates or CO produced by methoxide dehydrogenation with this “preadsorbed” oxygen, which was produced during the annealing treatment. Results obtained in previous studies of the reaction of methanol on oxygen-covered Pd surfaces, however, provide an argument against this scenario. Davis and Barteau used TPD to characterize the reaction of alcohols on oxygen-covered Pd(111)50 and observed that adsorbed oxygen atoms both stabilize alkoxides from dehydrogenation and react with them to form carboxylate species at temperatures below 240 K. Formate intermediates produced by reaction of methanol with adsorbed oxygen on Pd(111) decompose to gaseous CO2 and H2O at 280 K. While alloying of the Pd with Zn may have some effect on this reaction, the fact that the formate species on Pd decompose at a temperature ∼200 K below that observed for the low-temperature CO2 product in the present study strongly suggests that this is not the operative pathway for Pd/ZnO(0001). It appears more likely that the O required for oxidation of the CO, produced by dehydrogenation of methanol, to CO2 either migrates from the ZnO to the Pd during the TPD experiment or the reaction occurs at sites at the interface between the metal film or particle and the ZnO substrate. Thus, we propose the following mechanism for the production of CO2 on Pd/ZnO(0001)
300 K CH3OH + Pd f CO-Pd + 2H2(g)
(12)
400-550 K CO-Pd + ZnO f CO2(g) + PdZn
(13)
Reactions analogous to reaction 12 have been proposed as a step in the so-called redox mechanism for the water gas shift (WGS) reaction (i.e., CO + H2O f CO2 + H2) on group VIII metals supported on reducible oxides.51 For example, Bunluesin et al. have reported that the catalytic cycle for WGS on Pd/ CeO2, proceeds via reduction of ceria near the Pd-ceria interface by CO adsorbed on the metal.52 This reaction produces CO2 and an oxygen vacancy on the ceria, which is subsequently reoxidized by water. The results of this surface science study, therefore, suggest that in Pd/ZnO catalysts, Zn may act as a redox site that provides oxygen for reaction with CO adsorbed on the metal to produce CO2. These Zn sites could be located either at the Pd/ZnO interface or on the surface of the PdZn alloy. It should be noted, however, that it is unlikely that this is the pathway for the production of CO2 during MSR on Pd/ZnO since the equilibrium conversion for the MSR reaction under typical operating conditions would limit the selectivity to CO2 to values significantly less than that observed experimentally. Indeed, the high selectivity for CO2 (>95%) during MSR on Pd/ZnO suggests that the reaction does not proceed through CO as an intermediate. The reverse WGS reaction, however, may play a role in determining the overall selectivity. Thus, the reverse of the reactions observed in this study may still be important in MSR on Pd/ZnO.
Conclusions The TPD results obtained in this study are consistent with those reported in our previous study of the growth of vapordeposited Pd films on ZnO(0001), which showed that the Pd films follow a two-dimensional growth mode at 300 K and agglomerate into particles and incorporate Zn from the substrate upon annealing at temperatures above 700 K. The vapordeposited Pd was found to preferentially poison the activity of the ZnO(0001) surface for methanol oxidation. This result suggests that the metal islands nucleate at step edges on the ZnO(0001) surface, which are thought to be the location of coordinatively unsaturated cation-anion pairs that are the active sites for the dissociative adsorption of methanol on this surface. The supported Pd was also found to modify the selectivity for the decomposition of methanol-derived formate intermediates on exposed portions of the ZnO(0001) surface. On clean ZnO(0001), the ratio of CO to CO2 produced during formate decomposition was 3:1 and decreased to only 1.7:1 following deposition of 0.35 ML of Pd. The reaction of methanol on submonolayer and monolayer Pd films on ZnO(0001) was similar to that on bulk Pd undergoing complete dehydrogenation at room temperature to produce adsorbed CO and gaseous H2. Upon heating, the CO desorbed between 450 and 470 K, which is consistent with that reported previously for CO desorption from low index planes of Pd. Complete dehydrogenation of methanol to CO and H2 also occurred on PdZn alloy particles supported on the ZnO(0001) surface. The incorporation of Zn into the Pd, however, weakened the interaction of CO with the metal surface, causing the CO desorption temperature to decrease as the amount of Zn in the PdZn alloy increased. Finally, a pathway for the oxidation of CO to CO2 on Pd/ ZnO(0001) and PdZn/ZnO(0001) involving oxygen supplied by the ZnO support was observed. This reaction occurred either through migration of oxygen from the ZnO to the metal or at the metal-ZnO interface. The peak temperature for CO2 produced by this pathway was a function of the composition of the supported metal and decreased upon alloying of the Pd with Zn. Acknowledgment. We greatly acknowledge funding for this work provided by the U.S. Department of Energy (Grant No. DE-FG02-05ER15712) and would like to thank Dr. Yong Wang and Dr. Abhaya Datye for helpful discussions. References and Notes (1) Chin, Y. H.; Dagle, R.; Hu, J. L.; Dohnalkova, A. C.; Wang, Y. Catal. Today 2002, 77, 79. (2) Xia, G.; Holladay, J. D.; Dagle, R. A.; Jones, E. O.; Wang, Y. Chem. Eng. Technol. 2005, 28, 515. (3) Peters, R.; Dusterwald, H. G.; Hohlein, B. J. Power Sources 2000, 86, 507. (4) Emonts, B.; Hansen, J. B.; Jorgensen, S. L.; Hohlein, B.; Peters, R. J. Power Sources 1998, 71, 288. (5) de Wild, P. J.; Verhaak, M. J. F. M. Catal. Today 2000, 60, 3. (6) Lindstrom, B.; Pettersson, L. J. Int. J. Hydrogen Energy 2001, 26, 923. (7) Lindstrom, B.; Pettersson, L. J.; Menon, P. G. Appl. Catal., A 2002, 234, 111. (8) Jiang, C. J.; Trimm, D. L.; Wainwright, M. S.; Cant, N. W. Appl. Catal., A 1993, 93, 245. (9) Peppley, B. A.; Amphlett, J. C.; Kearns, L. M.; Mann, R. F. Appl. Catal., A 1999, 179, 21. (10) Twigg, M. V.; Spencer, M. S. Top. Catal. 2003, 22, 191. (11) Ritzkopf, I.; Vukojevic, S.; Weidenthaler, C.; Grunwaldt, J. D.; Schuth, F. Appl. Catal., A 2006, 302, 215. (12) Mastalir, A.; Frank, B.; Szizybalski, A.; Soerijanto, H.; Deshpande, A.; Niederberger, M.; Schomacker, R.; Schlogl, R.; Ressler, T. J. Catal. 2005, 230, 464.
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